A shaft seal of the aforesaid type is frequently used particularly in turbomachines which have a shaft which is guided out of the casing and enables the connection of a drive or driven unit. It lies within the nature of the shaft seal that on account of the relative movement of the shaft surface to the adjacent casing a one hundred percent seal-tightness cannot be achieved. Particularly in the case of toxic or explosive process fluids, which should be kept away from the environment by means of the shaft seal, leakages must be carefully drained off. Also, for example in the case of steam turbines or gas turbines, the process fluid is prevented from discharging into the environment by means of such a shaft seal and the leakage of the shaft seal or the amount of discharge by suction has a direct influence upon the resulting thermal efficiency. Minimizing the leakages of a shaft seal is one of the most important tasks within the scope of the design of such machines.
In the case of turbocompressors, so-called tandem gas seals frequently undertake the task of sealing the pressure chamber inside the casing in relation to the atmosphere. The tandem gas seals are contactless seals and are lubricated with dry, filtered sealing fluid or sealing gas.
A conventional arrangement with a shaft seal of the aforesaid type is schematically shown in FIG. 1. A shaft S extends through a penetration PT of a casing C. In the interior of the casing C, there is a process fluid PF under a sealing pressure PPF. The process fluid PF is delivered by means of a compressor CO at the sealing pressure PPF. Outside the casing C, there is air AM under an ambient pressure PAM. A gap G between the shaft S and the casing C in the region of the penetration PT is sealed by means of a shaft seal SS. The shaft seal SS comprises a plurality of sealing modules SM, inter alia two main seals, these being a first main seal MS1 and a second main seal MS2. The two main seals MS1, MS2 are designed as gas seals DG1, DG2 or dry gas seals DGS. Starting from the interior of the casing C, provision is first made for two labyrinth seals, these being a first labyrinth seal LS1 and a second labyrinth seal LS2, between which a pressure-increasing sealing fluid SFP is introduced. The labyrinth LS1 and the pressure-increasing sealing fluid SFP have the purpose of increasing the sealing pressure to an at least necessary pressure level and are required only if the existing pressure in the compressor is lower than the at least necessary pressure level. Between the first main seal MS1 and the second, outer labyrinth seal LS2, moreover, a sealing fluid is fed into the gap G. As a consequence of the feed of sealing fluid SF, a mass flow is created through the first main seal MS1 to the outside and a mass flow through the labyrinth seal LS2 in the direction of the compressor. This mass flow as a rule is relatively low and does not build up any appreciable differential pressure in the labyrinth LS2. The mass flow of the pressure-increasing fluid SFP is measured so that together with the mass flow of the sealing fluid SF which flows through the labyrinth LS2 a pressure differential builds up in the labyrinth LS1 which in addition to the pressure in the compressor corresponds to the at least required pressure level. This mass flow flows back into the interior of the casing C. Between the first main seal MS1 and the second main seal MS2, provision is made for a third labyrinth seal LS3. Between the third labyrinth seal LS3 and the second main seal MS2, an intermediate sealing fluid ISF is introduced into the gap G. Whereas the sealing fluid SF is a process fluid PF, the intermediate sealing fluid ISF is either an inert fluid or inert gas or the ambient medium, in most cases being nitrogen.
Between the first main seal MS1 and the second main seal MS2, specifically inward of the third labyrinth seal LS3, the mixture of sealing fluid SF and intermediate sealing fluid ISF, or process fluid and inert fluid or ambient fluid, which accumulates there, is drained off to a subsequent process which is not shown in more detail. The process can also be a torch by means of which the mixture is burned off. Outward of the second main seal MS2, an additional tandem arrangement of a labyrinth seal is frequently located, consisting of two seals LS4, LS5, between which a separation fluid SPF is introduced. A mixture of separation fluid SPF and intermediate sealing fluid ISF, which flows in the outward direction through the second main seal MS2 as leakage, is directed by means of a second drain EX2 to a process or also to a torch.
Beneath the sealing arrangement, FIG. 1 shows the pressure pattern over the axial direction, from which results the flow directions through the seals. The dry gas seals are not optionally reversible with regard to throughflow. In this respect, under specific operating conditions an increased amount of pressure-increasing sealing fluid SFP has to be supplied.
The arrangement of the gas seal which is shown in FIG. 1 is also referred to as a tandem gas seal. In the type of construction of the tandem gas seal with or without labyrinth between the two main seals, the intermediate sealing fluid is only required with the type of construction with labyrinth. An intermediate sealing fluid is normally nitrogen from an external source. Both partial amounts of the sealing fluid SF between the main seal MS1 and the additional shaft seal LS2 and partial amounts of the intermediate sealing fluid ISF between the main seal MS2 and an adjacent additional shaft seal LS3 are fed to the first drain EX1, wherein the pressures, as in the pressure pattern which is shown beneath the schematic arrangement in FIG. 1, are selected in such a way that the greater part of the fed fluid finds its way into the first drain EX1. A smaller part of the intermediate sealing fluid finds its way into the second drain EX2 through the second main seal MS2. The additional shaft seals LS4 and LS5 with the fed separation fluid SPF essentially serves for shielding the outer main seal MS2 against contamination of the environment AM, which may be contaminated as a result of, for example, oil mist from an adjacent bearing. The separation fluid discharges partly into the environment AM and it is partly drained off in the second drain EX2. For the sealing module SM, consisting of the additional shaft seals LS4, LS5, carbon rings or other types of seal can also be used.
A radial double seal arrangement of the type referred to in the introduction is already known from DE 20 2008 003 418 U1. A tandem arrangement of a dry gas seal is known from JP 2006 08 38 89 A and from U.S. Pat. No. 3,880,434. A simple arrangement of a radial double seal is known from U.S. Pat. No. 6,325,382 B1. The tandem arrangement of a radial double seal with the interposition of a labyrinth seal is already known from EP 1 914 387 A1.